Create a custom Wordpress search widget that limits results to one category

I am in need of a custom Wordpress widget that will restrict search results to a hidden category field that I can specify. The background is this: I have created a Glossary of financial terms, with each term being a post assigned to a category named Glossary. When the visitor clicks on GLOSSARY from the main menu (see attached screen capture), the search box on that page should only search on posts in the Glossary category. The default search for the rest of the site would remain as is? The widget should be designed so: 1. it takes its the search box and results display formatting from the theme 2. it won't be overwritten in a theme update. As far as implementation goes, if possible I would like you to upload the widget file(s) here and provide instructions on where on the host I should place it/them and any other steps required to implement the widget. Finally, I am using the Avada theme from Theme-Fusion. Thanks for your interest in my project.